If it's tart, my first reaction then is to whisk it into a vinaigrette and counter the tartness with some honey or sugar. This vinaigrette can then be used on salads, fish, shellfish (try making a salad with crab and avocado for this dressing), and with pork or chicken. You could also make a grapefruit mignonette and use it on fresh shucked oysters.
You can use also it as a base for a dipping sauce for won tons, egg rolls, rangoons, etc. Add it to either a sweet/sour based sauce, or to a fish sauce based recipe. You can even just use the vinaigrette as well. Just add maybe some ginger, lemongrass, thai basil, thai chiles, etc.
For the vinaigrette/mignonette, I would recommend using either champagne or sherry vinegar so that the flavor of the grapefruit can be accentuated, and not overwhelmed. Red Wine vinegar will also work but the flavor of the vinegar is stronger and may overpower the flavor of the grapefruit. Balsamic would be another option but you would have to really work on adjusting the flavors as you go along because of the definitive flavor that balsamic has.
